Pseudomonas aeruginosa is one of the leading causes of nosocomial infections throughout the world. P. aeruginosa nosocomial pneumonias are associated with higher mortality than other organisms, and ventilator- associated pneumonia has a mortality rate as high as 30%. P. aeruginosa infection is also the leading cause of death in cystic fibrosis patients. The increasing resistance of P. aeruginosa to antibiotics and the limited number of new antibiotics available for its treatment underscores the importance of understanding how to enhance the host response to P. aeruginosa, which is our long-term goal. A key component of the host response to acute lung infection by this pathogen is a robust recruitment of neutrophils, and we have determined that the protease ADAM17 is an important gatekeeper of neutrophil infiltration from the blood into the lungs during P. aeruginosa infection. ADAM17 is a membrane-associated protease constitutively expressed by leukocytes. Its activity is highly inducible upon leukocyte activation, and the protease regulates the density of cell surface adhesion molecules and the release of soluble pro-inflammatory factors. In our preliminary studies, gene-targeting ADAM17 in leukocytes markedly attenuated acute P. aeruginosa lung infection and bacteremia in mice. The overall objective of our study is to determine the mechanism by which ADAM17 exerts itself on the host response during acute P. aeruginosa lung infection. We hypothesize that overactivation of ADAM17 during severe infection promotes neutrophil dysfunction and pseudomonal pneumonia. In Aim 1 of this R21 (exploratory/developmental) grant application, we will determine the immunomodulatory role of ADAM17 during acute P. aeruginosa lung infection. To determine the underlying mechanism of enhanced bacterial clearance in conditional ADAM17 knockout mice, we will carefully evaluate neutrophil recruitment into the infected lung and airspace as well as local and systemic inflammation, as suggested by our preliminary findings. In Aim 2, we will explore for the first time the effects of a highly selective, small-molecule ADAM17 inhibitor in mice during acute pseudomonal pneumonia. We will evaluate therapeutic and prophylactic use of the inhibitor, and its effects on the host response. The impact of these studies is that they will critically advance our understanding of the mechanisms of action of ADAM17 during lung infection, as well as indicate the therapeutic potential of this immune regulator.